Desmos is a nice online graphing calculator for a browser. I posted earlier about it, but they have recently introduced a free iPad app for graphing calculator fans. You can click here to download the free app for your iPad.
On the iPad, you can graph on cartesian, parametric, and polar graphs. It has nice slider feature for variable graphs. You can easily zoom in or out on your graphs. You can graph inequalities, as well. You can also use it as a scientific calculator - even if you don't need a graph.
This app is very easy to use and intuitive to figure out. Give it a try.